1. Technical Field
The invention relates to a method for predetermining the composition of a gas flood used in an enhanced oil recovery process and, more particularly, for determining the minimum level of enrichment required to render a substantially immiscible gas miscible in a crude oil.
2. Description of Related Art
In a gas flood employing a condensing gas drive, intermediate hydrocarbon components in the injected gas condense upon contact with the crude oil in place at the formation temperature and pressure. The condensed hydrocarbon intermediates mix with the crude oil in situ, thereby altering the crude oil composition. If sufficient hydrocarbon intermediates condense from the gas and mix with the oil, the flooding gas and the oil in place ultimately become miscible. This effect is termed a miscible condensing gas drive and it substantially enhances oil recovery from the formation.
Many gases presently employed as flooding gases do not contain sufficient hydrocarbon intermediates to achieve miscibility with the crude oil at formation conditions, i.e., the flooding gases are substantially immiscible in the crude oil. In order to achieve miscibility in the crude oil, it is necessary prior to injection to artificially enrich the flooding gas, termed bulk gas hereafter, with hydrocarbon intermediates contained in an enriching fluid.
The resulting enriched gas is a multicomponent gas comprised of the bulk gas and the enriching fluid. The enriched gas has sufficient hydrocarbon intermediates to render it substantially miscible in the oil upon extended multiple contacting. Since the enriching fluid is often considerably more expensive than the bulk gas, it is desirable to achieve the miscible condensing gas drive using a minimum quantity of the enriching fluid. Thus, determination of the minimum level of enrichment required to render a substantially immiscible gas miscible in a crude oil in place is critical to the operation of a miscible gas flood.
The slim tube method is a widely accepted method for determining the minimum level of enrichment required to render a gas miscible in a crude oil. A slim tube is a long narrow tube approximately 12.2 to 18.3 meters long and having an inside diameter of 0.64 cm or less and packed with an unconsolidated material such as sand or glass beads. The tube is saturated with oil and thereafter flooded with a gas having a given level of enrichment and at constant pressure and temperature. The oil recovery is determined at that level of enrichment and then similar floods are conducted at different levels of enrichment. The oil recovery at each level of enrichment is measured as a function of the volume of gas injected. The oil recovery efficiency is determined thereafter as a function of the enrichment level. The minimum level of enrichment, as determined by the slim tube method, is the level of enrichment above which there is very little increase in oil recovery efficiency. The slim tube method is extremely time-consuming, taking several days to determine the minimum level of enrichment required for a single gas-crude oil system.
As such, an accurate and more rapid method is needed for determining the minimum level of enrichment required for a gas used in a miscible condensing gas drive.